1. Field of the Invention
The present invention relates to a cam mechanism for use in a camera for driving lens held by a movable lens-frame, and more particularly, to a cam mechanism in which cam members can be formed with a plastic material.
2. Description of the Related Art
As well known, in a conventional cam mechanism for driving movable lens in a zoom lens system, the rotation or the linear movement of a zoom control ring is transmitted to a movable lens-frame supporting the movable lens through the engagement of a pin and a cam slot or through a cam and a cam follower which has been brought in contact with the surface of the cam so that the movable lens-frame is moved along the optical axis.
Heretofore, the manufacturing cost of the lens-barrel of the zoom lens system was expensive because many manufacturing processes are required to finish the cam members with a high accuracy. In order to overcome the above-described disadvantage, various cam members which can be manufactured by a simple molding method have been proposed.
The cam member disclosed in Japanese Utility Model Publication No. 52-4924 is advantageous because it is manufactured by a single process by using a plastic material and a sintered alloy.
However, according to this prior art, a pin mounted on a lens-frame which moves along a linear slot parallel to the optical axis formed on a stationary cylinder fixed to a camera body is brought in contact with the cam face formed on the notch of the rotatable cam ring and the lens-frame moves along the optical axis according to the rotation of the cam ring. However, since the cam face is formed on the face of the notch defined by cutting off part of the cam ring, the strength of the cam ring is not favorable so that deformation thereof may result during the time when it is molded.
In addition, no description is made in the prior art with regard to the method for mounting the cam ring on the stationary cylinder.
It is difficult to assemble a conventional cam mechanism comprising a stationary cam ring fixed to a camera body on which a linear cam slot parallel to the optical axis is formed, a movable cam ring movable about the stationary cam ring, on which a movable cam slot not parallel to the optical axis is formed, and a cam pin which passes through the intersection point of the movable cam slot and the linear cam slot is mounted on a lens-frame, because it is necessary to insert the pin through the intersection point of the movable cam slot and the linear cam slot while positioning them.